Simplify removing leading and trailing separators

[cottsay]

This change is adapted from ament/ament_package#152, authored by @robwoolley.

Previously, we checked if the leading or trailing character was a colon and then used a wildcard to remove it.

It is simpler to just remove a leading or trailing colon. This has the added benefit of only using shell built-in functions.

Requires tests that are added in #713.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.98%. Comparing base (3310754) to head (5eab722).
⚠️ Report is 1 commits behind head on cottsay/shell-tests.

Additional details and impacted files

@@                 Coverage Diff                  @@
##           cottsay/shell-tests     #712   +/-   ##
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  Coverage                86.98%   86.98%           
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  Files                       69       69           
  Lines                     4088     4088           
  Branches                   706      706           
====================================================
  Hits                      3556     3556           
  Misses                     421      421           
  Partials                   111      111           

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[cottsay]

So this is sort of interesting, because I uncovered some issues in #713 that are solved by this PR, but not because of the lack of -c support for head or tail that originally motivated the change. It appears that on macOS, I have echo -n foo working as expected in zsh, but when I switch to sh, the /bin/echo executable that's used doesn't accept -n anymore so the conditional never fires.

It doesn't actually fail, it just doesn't remove the leading/trailing separators like it should.

Given that there's actually buggy behavior on a supported platform, I'm a little more motivated to get this pushed through and get #713 merged with some actual tests here.

[knmcguire]

I've tested out these solutions directly in a bash terminal and I can confirm this working.

Just a comment that it won't remove any double colons before and after the environment variable path string, nor does it remove any double colons in between. But I've learned that this is not really an edge case that has happened before, if at least nobody has reported this at least...